Since a polyproylene film has excellent appearance, heat resistance, rigidity, etc., the film is widely used in a packaging field of food packaging, fiber packaging, etc.
Recently, a high-speed film formation by a large-sized film forming machine has been practiced to improve the productivity and also various thicknesses of films have been employed to widen the purposes of uses. In this case, as a result thereof, a cooling condition at film formation changes, which gives large influences on the transparency of the film formed, and a problem occurs that the transparency of the film, which is good at a low-speed film formation or in the thin films having a thickness of from about 20 to 30 .mu.m, is greatly reduced at a high-speed film formation or in the films having a thickness of at least 50 .mu.m.
To overcome the problem, an attempt to decrease the surface temperature of a cooling roll by decreasing the temperture of cooling water, an attempt to directly cool a molten resin extruded with water without using a cooling roll, etc., have been made and an effect has been obtained to a certain extent. However, the influences by the temperature of cooling water, the deviation of the flow rate, etc., cannot be avoided and a sufficient effect has not yet been obtained.
Thus, development of a polypropylene composition and a polypropylene film having a good transparency showing a small film-forming condition dependency and a small thickness dependency of the transparency has been desired.
A reforming method of thermally decomposing or peroxide-decomposing a polypropylene resin, a method of compounding a polypropylene resin with a sorbitol nucleating agent, etc., are known as a method of improving the transparency of a polypropylene film.
In the reforming method of decomposing a polypropylene resin, the improvement effect may be obtained at a low-speed film formation when the surface temperature of a cooling roll is low, but no effect is obtained at a high-speed film formation when the surface temperature of a cooling roll is high. Further, in the case of a high-speed film formation of thick films, the transparency is greatly reduced.
Also, in the method of compounding a sorbitol nucleating agent, the effect of improving the transparency may be obtained, but since use of the sorbitol nucleating agent causes an unpleasant odor of the film obtained, there is a restriction on the use thereof.